【作者】 张轲；

【导师】 程铁欣；

【作者基本信息】 吉林大学 ， 物理化学， 2009， 博士

【摘要】 本文制备了一系列Ni基六铝酸盐催化剂,通过XRD、DTA-TGA、DRS UV-Vis、XPS、TEM等技术对催化剂晶体结构、金属离子间相互作用等物理化学特征进行了较深入的研究,考察了催化剂的反应性能,获得了有意义的研究成果。主要包括:1.对于LaNixAl11O19-δ催化剂来说,只有当x≥0.5(Ni/La摩尔比)时,才能得到纯净的六铝酸盐晶相结构,这部分Ni2+对晶相的稳定起到至关重要的作用。而当x>1时,有尖晶石型NiAl2O4和钙钛矿型LaAlO3等杂相生成,从而破坏了催化剂的晶相结构。因此,x应控制在0.5-1.0之间,才能使催化剂形成很好的六铝酸盐晶相结构。2. Pr取代La进行A位调变的六铝酸盐催化剂La1-xPrxNiAl11O19-δ的晶相结构没有发生改变。但Pr的引入使还原后的催化剂表面活性Ni0增多;同时, Pr的掺入促进了La离子与Ni离子之间的电子转移,使Ni能保持在Nio的活性状态,从而促进了反应在催化剂表面的进行。3.对于不同稀土金属A位调变的六铝酸盐催化剂La0.8A0.2NiAl11O19-δ,由于A位调变对晶相结构中Ni2+产生影响不同,导致催化活性的改变。其中,由于Pr与La具有相似的半径而具有最佳的结构作用,因此使得活性最佳;而活性最差的Ce,则是因为在反应过程中本身存在有Ce3+/Ce4+氧化还原循环,影响了它与La和Ni之间的相互作用,减少了La与Ni之间的相互作用,从而降低了其反应活性。4.对催化剂表面积碳规律进行了系统的研究。根据积碳氢化温度的高低将积碳分为三种类型:Cα,Cβ和Cγ。Cα为易被CO2消除的活性碳,而Cβ和Cγ为不易被CO2消除的非活性碳,它们之间存在一个生成沉积并逐渐聚集的转化过程。催化剂经氢气还原后(或在反应过程中)表面吸附活泼氢,形成··O···H类似物促进了反应气中CO2的活化解离交换生成活性氧,能够快速消除催化剂表面活性碳,阻止其进一步聚集形成非活性碳,促使催化剂表面达到积碳消碳的平衡,保持了催化剂反应活性的稳定。更多还原

【Abstract】 In recent years, CO2 reforming of methane to synthesis as a new approach of CH4 conversion and CO2 utilization has gained growing attention from academia and industry. Traditional supported nickel catalysts have also been reported to be effective catalyst for the title reaction and have been studied extensively. However, they suffer from serious deactivation due to carbon deposition, nickel particle sintering and phase transformation, resulting in rapid decrease in activity and lifetime. Therefore, it is very important to improve the state of nickel catalyst in order to overcome these disadvantages. The LaNiAl11O19-δcatalyst prepared exhibits rather high catalytic activities for CO2 reforming methane and also possesses essential advantages over traditional supported nickel catalysts in resistance to carbon deposition, and it is believed that metallic nickel (Ni0) is the active component for this reaction. In order to investigate the action of metallic Ni0 reduced from the lattices and Ni2+ located in the lattices during the process of the crystalline formation and the reaction, a series of catalysts, such as LaNixAl11O19-δ,La1-xPrxNiAl11O19-δ, and hexaaluminate La0.8A0.2 NiAl11O19-δcatalysts (A: rare earth metal), were prepared. Their catalytic activities for this reaction were investigated and their structures were characterized in detail. The behavior of carbon deposition on the Ni substituted hexaaluminate catalysts(LaNi Al11O19-δ) studied also. Some significative results are obtained.It is found that the unique MP phase can be formed when the ratio of Ni/La (mol) of inserted Ni2+ions is higher than 0.5. This part of Ni2+ions, playing very important role in the stability of the MP crystals, is very difficult to be reduced from the lattices. At the same time, when the ratio of Ni/La (mol) of inserted Ni2+ions is lower than 0.5, the Ni2+ locate in tetrahedral sites at first. When the mount of inserted Ni2+ions is higher, more Ni2+ locate in octahedral sites. This may be one of the reasons for the catalytic activity of these catalysts.Based on the experimental data, we analyzed the main reactions, which may take place under the experimental conditions and assumed the mechanism of CO2 with CH4 over the hexaaluminate LaNiAl11O19-δcatalyst in detail. At low temperature CH4→C+2H2 reaction is predominant. However, at high temperature CH4+CO2→2CO+2H2 is main reaction. While the main reactions of CO2 with CH4 over hexaaluminate LaNiAl11O19-δcatalyst is CH4+CO2→2CO+2H2.A series of Pr-doped hexaaluminate La1-xPrxNiAl11O19-δ(x=0,0.2,0.4,0.6,0.8,1.0) as catalysts for carbon dioxide reforming of methane were prepared. These catalysts were studied in more detail and it was found that the addition of Pr can promote the interactions of the Ni ions and the La ions and reduce the particle size of the metal and improve the dispersion of the Nio on these catalysts after reduction and more Ni2+ can be reduced to Nio from the crystal structure of hexaaluminates. A series of A-modified hexaaluminates, La0.8A0.2NiAl11O19-δ(A = Ce,Pr,Nd,Sm) as new catalysts for carbon dioxide reforming of methane to synthesis gas, were prepared by decomposition of nitrates and calcination at high temperature. Nickel ions as active component were inlayed in the hexaaluminate lattices to substitute part of Al ions. The series of catalysts exhibited significantly catalytic activity and ability of restraining carbon deposition at high temperature, for instance at 750 oC for 2h. The experimental results indicate that the catalytic properties of the La0.8Pr0.2NiAl11O19 are more optimal than others. These catalysts were studied in more detail and it was found that the active nickel phase strongly depends on the interactions of the Ni ion and the La ion, which are related to the additives.The reduced Ni-based hexaaluminates exhibited rather high activity of decomposition of methane. It is concluded that there are three kinds of deposited carbon: Cα,Cβand Cγon the surface of the catalyst after the top reaction. And based on the analysis of XPS, the Cαis attributed to carbide carbon and Cβand Cγare attributed to graphitic carbon. At the same time , the decomposition of methane is the main reason for the deposited carbon on the surface of the catalysts in the top reaction. Finally, the author proposed that a mechanism of carbon deposition and elimination in the reaction over the hexaaluminates catalyst. It is assumed that, in the case of hexaaluminates catalyst (eg.LaNiAl11O19-δ), as a first step the active H is absorbed on the catalyst after reduction. Then there is··O···H exist on the surface of the hexaaluminates catalyst. During the CH4-CO2 refoming reaction, it promotes the activated dissociation of CO2, which produce the active oxygen. Whereafter the active oxygen eliminates the active carbon species which is formed on the metallic nickel surface via the activation of CH4 before converting to graphite carbon.更多还原